In recent years, a Reconfigurable Optical Add-Drop Multiplexer (ROADM) has been put into practical use in order to realize a large-capacity flexible optical network. The ROADM is provided, for example, in respective nodes of a WDM transmission system. The ROADM can drop an optical signal of a desired wavelength channel from a WDM optical signal, and can guide the optical signal to a client. The ROADM can also add a data signal received from a client to a WDM optical signal.
In order to further increase a capacity of an optical network and/or to improve flexibility of the optical network, a scheme of further efficiently utilizing communication resources (here, a frequency or a wavelength) has been discussed. As an example, multicarrier modulation for multiplexing a plurality of subcarrier optical signals has been discussed. As one example of multicarrier modulation, Orthogonal Frequency Division Multiplexing (OFDM) has been put into practical use. In the description below, an optical signal into which a plurality of optical signals having different optical frequencies or wavelengths are multiplexed may be referred to as a “multicarrier signal” or a “multiplexed optical signal”.
An optical orthogonal frequency division multiplexing communication device is described, for example, in Japanese Laid-open Patent Publication No. 2011-109439. In addition, Japanese Laid-open Patent Publication No. 2011-215603 describes an optical signal processing device that enables information transmission in an arbitrary location of an optical network.
A technology for processing a wavelength with very fine granularity is needed to transmit an arbitrary subcarrier optical signal in a multicarrier signal to a desired destination. However, it is difficult to implement a wavelength selective switch that has steep transmission characteristics. Stated another way, it is not easy to individually process subcarrier optical signals in a multicarrier signal in an existing technology. Accordingly, in an existing technology, it is difficult to sufficiently reduce a frequency spacing (or a wavelength spacing) between channels/subchannels implemented in an optical network. Note that this problem is not limited to a multicarrier signal into which a plurality of subcarrier optical signals are multiplexed, and this problem may occur in a multiplexed optical signal (for example, a wavelength division multiplexed optical signal) into which a plurality of optical signals are multiplexed.